The present invention relates to a strip line cable comprising one or more pairs of electrical conductors arranged in a spaced, parallel relationship with each element of each pair of conductors being at opposite positions on either side of a dielectric tape, this assembly being encased between layers of non-porous plastic. More particularly, this invention concerns a strip line having excellent signal transmission performance and dimensional stability, and having resin materials that can be easily stripped away for termination.
It has been widely known that the use of materials having low dielectric constants, dielectric loss, and frequency dependence in transmission lines can achieve improved electric signal transmission performance. A lower dielectric constant of the dielectric in a strip line allows for a smaller line size and a faster signal propagation speed with the same characteristic impedance as larger lines have. Moreover, if the dielectric constant and the dielectric loss is less dependent on signal frequency, the wave form of a pulse signal is less deformed.
The dielectric material to be used in a strip line should, therefore, have a low dielectric constant and dielectric loss with low frequency dependence. Continuously porous, crystalline, polymeric materials having a microstructure of numerous fine nodes interconnected by fibrils with numerous void spaces or pores between the nodes and fibrils have been known to be one such material meeting the above requirements. Various known porous dielectric materials are produced, for example, by stretching, by mixed filler extraction or a fiber papering method, from such polymers as polyolefins (such as polyethylene, polypropylene, polystyrene), nylon, polyesters, and fluorocarbons (such as polytetrafluoroethylene, fluorinated ethylene propylene, perfluoroalkoxy resin, and ethylenetetrafluoroethylene resin). A typical example of such a material is an expanded porous polytetrafluoroethylene (hereinafter, PTFE) produced by the method described in Japanese Patent Publication Sho 51-18991 (corresponding to U.S. Pat. No. 3,953,566). However, this expanded porous PTFE is highly flexible and dimensionally unstable and, hence, it has heretofore been used as a dielectric in coaxial cables by wrapping it around a conductor. Nevertheless, it was thought to be unsuitable as a dielectric in a strip line having the construction mentioned above.
In spite of the above-mentioned disadvantage, the inventor pursued expanded porous PTFE in a strip line since it possesses all the characteristics (low dielectric constant and dielectric loss with less frequency dependence) required for the dielectric material in a strip line, and performed various experiments to determine how such a material could meet the dimensional stability requirements for a strip line dielectric while simultaneously maintaining the advantages of the material itself. After a series of experiments, the inventor has provided the improved strip line construction of the present invention.